U.S. patent number 8,841,738 [Application Number 13/632,205] was granted by the patent office on 2014-09-23 for mems microphone system for harsh environments.
This patent grant is currently assigned to Invensense, Inc.. The grantee listed for this patent is Invensense, Inc.. Invention is credited to Jia Gao, Kieran P. Harney, Aleksey S. Khenkin.
United States Patent |
8,841,738 |
Harney , et al. |
September 23, 2014 |
MEMS microphone system for harsh environments
Abstract
A MEMS microphone system suited for harsh environments. The
system uses an integrated circuit package. A first, solid metal lid
covers one face of a ceramic package base that includes a cavity,
forming an acoustic chamber. The base includes an aperture through
the opposing face of the base for receiving audio signals into the
chamber. A MEMS microphone is attached within the chamber about the
aperture. A filter covers the aperture opening in the opposing face
of the base to prevent contaminants from entering the acoustic
chamber. A second metal lid encloses the opposing face of the base
and may attach the filter to this face of the base. The lids are
electrically connected with vias forming a radio frequency
interference shield. The ceramic base material is thermally matched
to the silicon microphone material to allow operation over an
extended temperature range.
Inventors: |
Harney; Kieran P. (Andover,
MA), Gao; Jia (Winchester, MA), Khenkin; Aleksey S.
(Nashua, NH) |
Applicant: |
Name |
City |
State |
Country |
Type |
Invensense, Inc. |
San Jose |
CA |
US |
|
|
Assignee: |
Invensense, Inc. (San Jose,
CA)
|
Family
ID: |
50384375 |
Appl.
No.: |
13/632,205 |
Filed: |
October 1, 2012 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20140091406 A1 |
Apr 3, 2014 |
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Current U.S.
Class: |
257/416; 257/777;
257/778 |
Current CPC
Class: |
H04R
1/086 (20130101); H04R 19/04 (20130101); H04R
19/005 (20130101); H04R 31/00 (20130101); H01L
2224/48137 (20130101); H01L 2224/73265 (20130101); H01L
2224/48227 (20130101); H01L 2224/48091 (20130101); H01L
2224/48091 (20130101); H01L 2924/00014 (20130101) |
Current International
Class: |
H01L
29/84 (20060101) |
Field of
Search: |
;257/416,777,778 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2252077 |
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Nov 2010 |
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EP |
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WO 2005/036698 |
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Apr 2005 |
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WO |
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Other References
Kopola et al., "MEMS Sensor Packaging Using LTCC Substrate
Technology", Proceedings of SPIE, vol. 4592, pp. 148-158, 2001.
cited by applicant.
|
Primary Examiner: Green; Telly
Assistant Examiner: Hillman; Damian A
Attorney, Agent or Firm: Imam; Maryam IPxLaw Group LLP
Claims
What is claimed is:
1. A microphone system in an integrated circuit package comprising:
a package base including opposing first and second faces, the
package base including a cavity in the first face; a first metal
lid coupled to the first face of the package base covering the
cavity and forming an acoustic chamber, the package base including
an aperture through the second face of the package base for
receiving an audio signal into the acoustic chamber; a filter
covering the aperture in the second face of the package base; a
silicon microphone secured to the package base about the aperture
within the acoustic chamber; and a second metal lid enclosing the
second face of the package base and the filter, the second metal
lid electrically connected to the first metal lid, the second metal
lid including an opening for allowing the audio signal to reach the
acoustic chamber.
2. The microphone system as defined by claim 1 wherein the package
base material is a ceramic.
3. The microphone system as defined by claim 1 wherein the package
base material is chosen from a group consisting of alumina and
aluminum nitride.
4. The microphone system as defined by claim 1 wherein the filter
is attached to the package base by the second metal lid.
5. The microphone system as defined by claim 1 wherein the second
metal lid includes the filter.
6. The microphone system as defined by claim 1 wherein the filter
comprises a polyetheretherketone fabric.
7. The system as defined by claim 1 wherein the filter is chosen
from a group consisting of a polytetrafluoroethylene fabric and a
sintered metal mesh.
8. The microphone system as defined by claim 1 wherein the filter
has a controlled acoustic impedance.
9. The microphone system as defined by claim 1 further comprising a
processing element secured within the acoustic chamber, the
processing element electrically communicating with the silicon
microphone.
10. The microphone system as defined by claim 1 further comprising
in-line package leads side-brazed to a lead frame.
11. The microphone system as defined by claim 1 further comprising
J-lead package leads.
12. The microphone system as defined by claim 1 further comprising
gull-wing package leads.
13. The microphone system as defined by claim 1 where the
integrated circuit package is surface-mounted.
14. The microphone system as defined by claim 1 wherein the first
metal lid and the second metal lid are connected with vias, where
the vias are spaced such that radio frequency interference is
reduced.
15. A method of forming a microphone system, the method comprising:
providing a package base including opposing first and second faces,
the base including a cavity in the first face, vias, and an
aperture through the second face; securing a MEMS microphone about
the package base aperture; securing a first metal lid to the first
face of the package base to form an acoustic chamber including the
cavity, wherein the acoustic chamber contains the microphone;
attaching a filter to the second face of the package base wherein
the filter covers the aperture; and attaching a second metal lid to
the second face of the base; and electrically connecting the vias
to each of the first metal lid and the second metal lid such that
the first metal lid and the second metal lid are electrically
connected.
16. The method of claim 15 wherein the package base is ceramic.
17. The method of claim 15 wherein the first and second metal lids
are attached to the package base by seam welding.
18. The method of claim 15 wherein the first and second metal lids
are attached to the package base by soldering.
19. The microphone system as defined by claim 1 wherein the silicon
microphone being positioned between the cavity and the filter.
20. The microphone system as defined by claim 1 wherein the silicon
microphone being secured to the package base about the aperture
within the acoustic chamber.
21. The microphone system as defined by claim 1 wherein the silicon
microphone including a diaphragm and an electrode capacitively
coupled together.
22. The microphone system as defined by claim 1 wherein the filter
being positioned between the aperture and the second metal lid.
23. The microphone system as defined by claim 1 wherein the filter
being positioned in a manner such as to avoid contamination.
24. The microphone system as defined by claim 1 further including a
microphone die situated between the cavity and the aperture and
implemented as an application specific integrated circuit (ASIC),
the ASIC being attached to the first face of the package base
within the cavity.
25. The microphone system as defined by claim 1 wherein the first
and second metal lids form an EMI shield.
26. The method of claim 15 wherein the filter being positioned
sufficiently away from the microphone to avoid contamination
thereof.
Description
TECHNICAL FIELD
The invention generally relates to microphones and, more
particularly, the invention relates to packaged microphones for
harsh environments
BACKGROUND ART
Microelectromechanical system ("MEMS") condenser microphone dies
typically have a diaphragm that forms a variable capacitor with an
underlying backplate. Receipt of an audio signal causes the
diaphragm to vibrate and generate a variable capacitance signal
representing the audio signal. This variable capacitance signal can
be amplified, recorded, or otherwise transmitted to another
electronic device. Such microphones typically are enclosed in a
package to shield sensitive microphone components from
environmental conditions.
These microphone packages have an aperture that allows acoustic
signals to reach the microphone. Undesirably, in addition to
allowing access for acoustic signals, the aperture can allow access
to contaminants that can compromise microphone operation. For
example, particles and moisture can enter the interior of the
microphone and interfere with movement of the variable capacitor.
Additionally, thermal expansion mismatches between the microphone
die and the package materials on which the microphone die is
mounted can limit the operating temperature range for these
microphone systems. These problems often compromise condenser
microphone systems operating in harsh environments having very
high-temperatures, radio frequency interference/electromagnetic
interference ("RFI"/"EMI"), and high volumes of airborne
contaminants such as dirt, dust, and liquids.
SUMMARY OF EMBODIMENTS OF THE INVENTION
In a first embodiment of the invention there is provided a silicon
MEMS microphone system in an integrated circuit package for use in
harsh environments. The microphone system includes a package base
which includes opposing first and second faces. The package base
has a cavity in its first face and a first metal lid coupled to the
first face of the package base covering the cavity to form an
acoustic chamber. The package base includes an aperture through the
second face of the package base for receiving audio signals into
the acoustic chamber. A MEMS microphone is secured to the package
base about the aperture within the acoustic chamber. A filter
covers the aperture in the second face of the package base to
prevent contaminants from reaching the microphone. A second metal
lid encloses the second face of the base and the filter; includes
an opening for allowing the audio signal to reach the acoustic
chamber; and is electrically connected to the first metal lid,
forming an EMI shield.
In some embodiments of the invention, the package base is a ceramic
material providing a good thermal expansion match for the silicon
microphone and any silicon processing elements attached to the
package base. In preferred embodiments, the metal lids are
connected with vias whose number and spacing reduces RFI that could
affect microphone signals. The filter material may be a fabric with
a controlled acoustic impedance and may be chosen from a
polyetheretherketone fabric or a polytetrafluoroethylene fabric or
an equivalent, in preferred embodiments of the invention. The
microphone components may communicate with other system components
external to the integrated circuit package by various means
including side-brazed in-line leads, J-leads, gull-wing leads or by
surface mounting the integrated circuit package on a circuit board,
according to various embodiments of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing features of embodiments will be more readily
understood by reference to the following detailed description,
taken with reference to the accompanying drawings, in which:
FIG. 1 schematically shows a cross-sectional view of a microphone
system configured in accordance with illustrative embodiments of
the invention;
FIG. 2 schematically shows a cross-sectional view of the microphone
in the system of FIG. 1;
FIG. 3 schematically shows a top view of the microphone system of
FIG. 1, without the lid; and
FIG. 4 illustrates a process for fabricating the microphone system
of FIG. 1 in accordance with illustrative embodiments of the
invention.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
In an illustrative embodiment of the present invention, a
microphone system is specially configured for use in harsh
environments. For example, the microphone system should be capable
of substantially normal operation in unusual and extreme
situations, such as those with extreme temperature swings, high
humidity and moisture (e.g., nautical environments), high shock
(e.g., military environments), and very dry conditions (e.g.,
deserts). The system thus may be considered to be a "ruggedized"
microphone system.
To that end, in summary, the microphone system may have a base with
a cavity, and a solid metal lid covering the cavity forming an
acoustic chamber containing a silicon microphone (i.e., a MEMS
microphone). The package base includes an aperture from the
acoustic chamber through the opposing face of the base for
receiving audio signals into the chamber. To prevent contaminants
from entering the acoustic chamber, the system also includes a
filter covering the aperture opening in the opposing face of the
base. A second metal lid encloses the opposing face of the base. To
protect the silicon microphone from electromagnetic interference
(a/k/a "EMI"), vias or other electrical interconnectors
electrically connect the lids to form an EMI shield (also referred
to as an "RF interference shield"). Finally, the base material
preferably is thermally matched to the silicon microphone to permit
use across a wide range of temperatures. Details of various
illustrative embodiments are presented below.
FIG. 1 schematically shows a cross-sectional view of a microphone
system 10 according to an illustrative embodiment of the invention.
As known by those in the art, the microphone system (also known as
a "packaged microphone") may be mounted to a larger system, such as
a printed circuit board of a larger system. For example, the larger
system may be a mobile telephone, and the printed circuit board may
position the microphone system 10 near an opening in its external
housing to receive a voice signal. The printed circuit board also
may have many components other than the microphone, such as power
circuitry, filters, etc., that may or may not directly control the
microphone system 10. In fact, the system printed circuit board may
be a daughterboard that is secured to a larger printed circuit
board (i.e., a motherboard).
The microphone system 10 shown may be considered to be a top port,
cavity down design. This "top port" designation is due to the fact
that its port 90 (see below for details of the port) is on the side
of the package that, when secured to the system printed circuit
board, is farthest from the board--it is on the top of the package
from that perspective. In a corresponding manner, the "cavity down"
designation is due to the fact that the cavity 40 (also discussed
below) is concave from the perspective of the printed circuit board
when mounted.
To those ends, the microphone system 10 includes a package base 20
forming the noted cavity 40, and a solid metal lid 30 secured over
the cavity 40. This connection of the lid 30 over the cavity 40
forms an acoustic chamber. The base 20 forms an aperture 50 through
the opposing faces for entry of an acoustic signal (e.g., a sound
signal) into the acoustic chamber.
A silicon microphone 70 (i.e., a MEMS microphone) is attached to
the first face of the base about the aperture 50. FIG. 2
schematically shows a cross-sectional view of the silicon
microphone 70 in accordance with illustrative embodiments of the
invention. Among other things, the microphone 70 includes a static
backplate 71 that supports and forms a variable capacitor with a
flexible diaphragm 72. In illustrative embodiments, the backplate
71 and diaphragm 72 each are formed from single crystal silicon
(e.g., the top layer of a silicon-on-insulator wafer, discussed
below). Alternatively, the diaphragm 72 is formed from a film of
silicon-based material, such as polysilicon, silicon carbide, or
silicon germanium. In a similar manner, other types of materials
can form the backplate 71. For example, the backplate 71 can be
formed from a relatively low temperature film, such as silicon
germanium. If thermal budget is not a primary issue, the backplate
71 can be formed from high temperature materials, such as
polysilicon, silicon carbide, or silicon germanium.
To facilitate operation, the backplate 71 has a plurality of
through-hole apertures ("backplate apertures 73") that lead to a
backside cavity 40. Springs 74 movably connect the diaphragm 72 to
a static/stationary portion of the microphone 70, which includes a
substrate that at least in part includes the backplate 71. The
springs 74 effectively form a plurality of openings that permit at
least a portion of the audio signal to pass through the microphone
70. These openings may be any reasonable shape, such as in the
shape of a slot, round hole, or some irregular shape.
The aperture 50 thus forms a top port for the microphone system 10.
The microphone system 10 thus has an enlarged back volume, which
primarily includes the acoustic chamber formed by the cavity 40 and
the lid 30. As known by those in the art, this enlarged back volume
enhances performance of the entire system 10. Moreover, to prevent
contaminants from reaching sensitive microstructure forming the
variable capacitor of the microphone, the aperture opening 50
preferably is covered by a filter 60. The filter should be selected
to permit easy access to desired acoustic signals while mitigating
the potential of access of undesired particles.
A second metal lid 80 is attached to the opposing face of the base
20. The second metal lid includes an opening 90 to allow sound to
reach the aperture 50 opening. In some embodiments, this second
metal lid has a raised portion that forms a front volume over the
face to which it is mounted. Such a front volume is optional,
however, and thus, may be omitted.
In illustrative embodiments, the first and second metal lids 30 and
80 are electrically connected to form an EMI shield about the
microphone die 70. The two metal lids 30 and 80 at least in part
form a Faraday cage that should mitigate the impact of EMI.
Accordingly, the microphone system 10 has an electrical
interconnection between the two lids 30 and 80, ensuring they are
at substantially the same electric potential. To that end, the
system shown in FIG. 1 has a plurality of vias 85, forming an
enclosure that reduces EMI that could inject noise into microphone
signals.
In some embodiments, the microphone system 10 has a specialized
circuit die 110 that controls operation of the microphone die 70.
To that end, the die 110 may be implemented as an application
specific integrated circuit ("ASIC 110") attached to the first face
of the package base 20 (i.e., within the cavity 40). Wire bonds 120
connect this circuit element die 110 to the microphone 70, and one
or both of the microphone die 70 and ASIC 110 to pads on the base
20. FIG. 3, a top down view of the microphone system without the
package lid 80, shows the pads 115. The pads 115 also attach the
wire bonds 120 (and, consequently the microphone 70 and ASIC 110)
to the interconnects (discussed below) that connect the system 10
to the noted system printed circuit board.
FIG. 2 also shows vias 85 that connect the metal lids 30, 80. The
number and spacing of the vias 85 can be chosen such that EMI can
be suppressed by the combination of the two metal lids and the
vias.
The microphone system 10 can use any of a wide variety of
electrical interconnection systems to electrically and mechanically
connect with an underlying system circuit board or base. For
example, FIG. 1 shows a plurality of leads 100 extending from the
base to allow the microphone die 70 (and ASIC die 110, described
below) to communicate with other electronic elements (not shown).
As shown in FIG. 1, such leads 100 may be side brazed in-line leads
for connecting directly to a circuit board (or to a socket).
Alternatively, the leads may be J-leads or gull-wing leads (not
shown) for surface mounting the microphone system package to a
circuit board. In yet other embodiments, the system 10 can have
pads (not shown) for surface mounting with the underlying
system.
The package base 20 material may be chosen so that its coefficient
of thermal expansion ("CTE") closely matches the CTE of the
microphone 70 and that of the ASIC 110. Accordingly, mitigating the
CTE mismatch should mitigate thermal stress between the base 20 and
microphone system elements, favorably expanding the operating
temperature range of the overall microphone system. In preferred
embodiments, the package base material is a ceramic, which has a
CTE that is close to that of silicon. The ceramic material for the
base may be chosen from a variety of ceramic materials including,
by way of example but not for limitation, aluminum nitride and
aluminum oxide (alumina). The ceramic package base may be
fabricated by a high temperature co-fired ceramic process, by a low
temperature co-fired ceramic process, or by another process as is
known in the art.
In other embodiments of the invention, materials other than
ceramics may be used for the package base, such as a plastic or a
composite material, e.g., FR-4 epoxy laminate. In preferred
embodiments of the invention using a ceramic package base, the
operating temperature range of these microphone systems can extend
from about -55 degrees C. to about 215 degrees C. and beyond.
The above noted filter 60 should seal the acoustic chamber from
many contaminants. To that end, the filter can be attached to the
opposing face of the package base 20 with adhesive, such as, for
example, an epoxy adhesive. In some embodiments of the invention,
the filter is attached to the package base by a form-fitting second
metal lid 80, either with or without adhesive applied to the
package base. The filter material may be chosen from any of the
high temperature acoustic fabrics that are known in the art. These
fabrics provide a controlled acoustic impedance and may include,
among other things, polyetheretherketone ("PEEK") fabric,
polytetrafluoroethylene ("PTFE") fabric, or their equivalents. The
filter may also be formed in the second metal lid 80. For example,
the filter may be formed by sintering. The filter may also be
formed in the package base, for example, by sintering.
It should be noted that discussion of a top port, cavity down
package is but one of a number of different potential embodiments.
For example, some embodiments may use a bottom port design, with
the cavity either up or down. As another example, some embodiments
may have a cavity up design with either a top or bottom port
design. Moreover, rather than solid metal lids 30 and 80, some
embodiments may be formed from another material, or a composite,
layered set of materials. For example, some embodiments may use
metal plated lids, conductive or nonconductive plastic lids, or
plastic lids with metal layers. The type of lid and configuration
depends on the intended application.
In fact, some embodiments may omit one of the lids 30 and 80.
Alternatively, other embodiments may use one lid formed from one
set of materials (e.g., a solid metal lid) and another lid 30 or 80
formed from another set of materials (e.g., a plastic lid).
Accordingly, discussion of solid metal lids 30 and 80 is for
illustrative purposes only and is not intended to limit all
embodiments.
Fabrication Process for MEMS Microphone System
FIG. 4 shows a process of fabricating the microphone system 10 in
accordance with illustrative embodiments of the invention. It
should be noted that for simplicity, this described process is a
significantly simplified version of an actual process used to form
the microphone system 10. Accordingly, those skilled in the art
would understand that the process may have additional steps not
explicitly shown in FIG. 4. Moreover, some of the steps may be
performed in a different order than that shown, or at substantially
the same time. Those skilled in the art should be capable of
modifying the process to suit their particular requirements.
The process begins at step 210, which provides a package base 20
having opposing first and second faces with the base. As shown in
FIG. 1 and discussed above, the base has the cavity 40 in the first
face, vias 85, and the aperture 50 through its second face. The
process then secures both 1) the MEMS microphone 70 in the cavity
about the package base aperture (step 220), and 2) the ASIC 110
next to the microphone in the same cavity. Among other ways, the
process may use a conductive or insulative adhesive to secure
bottom sides of both chips to the cavity. Wire bonds 120 may be
placed to connect ASIC 110 to the microphone 70, and one or both of
the microphone die 70 and ASIC 110 to pads 115 on the base 20.
Next, the process attaches the first metal lid 30 (step 230) to the
first face of the package base 20 to form the acoustic chamber,
which contains the microphone. In addition, this step also
electrically and mechanically connects the first metal lid 30 with
the vias 80. A filter 60 then is attached (step 240) to the
opposing face of the package base, covering the aperture 50. The
process then may secure second metal lid 80 (step 250) to the
opposing face of the base in a manner that mechanically and
electrically connects with the vias (step 260). Accordingly, this
also electrically connects both lids, forming the EMI shield. Among
other ways, the lids may be attached to the package base by low
temperature soldering using solder such as a tin-silver-copper
alloy. Alternatively, attachment of the lids may be accomplished
with seam welding, or by using a conductive adhesive.
Accordingly, illustrative embodiments permit use of a MEMS
microphone in a wider variety of environments. The CTE matched base
and dies enables a wider temperature variation, while the opposing
lids provide EMI protection and a more robust package. Moreover,
the filter further limits particle access to the sensitive
microstructure of the MEMS variable capacitor.
The embodiments of the invention described above are intended to be
merely exemplary; numerous variations and modifications will be
apparent to those skilled in the art. All such variations and
modifications are intended to be within the scope of the present
invention as defined in any appended claims.
* * * * *